Microstrip patch antennas have several well known advantages over other antenna structures. These antennas generally have a low profile and conformal nature, are lightweight, have low production cost, are robust in nature and compatible with microwave monolithic integrated circuits (MMICs) and optoelectronic integrated circuits (OEICs) technologies. However, one drawback of such devices is their relatively narrow bandwidth. In order to achieve wider bandwidth, a relatively thick substrate must be used. However, the antenna substrate supports tightly bound surface wave modes which represent a loss mechanism in the antenna. The loss due to surface wave modes increases as the substrate thickness is increased. It is desirable to develop conformal microstip antennas which enjoy wide bandwidth, yet do not suffer from the loss of attractive features of the conventional microstrip patch antenna.
One way to reduce the element-to-element mutual coupling is to surround the patch elements with metal walls. This technique effectively prevents surface wave modes from being excited in a substrate, thus allowing the substrate's thickness to be increased without serious effects. In addition to the common techniques of increasing patch height and decreasing substrate permittivity, a conventional method uses parasitic patches in another layer (stacked geometry). However, this has the disadvantage of increasing the thickness of the antenna. Parasitic patches can also be used in the same layer (coplanar geometry); however, this undesirably increases the lateral size of the antenna and is not suitable for antenna array applications.
As previously mentioned, a disadvantage of microstrip patch antennas which has limited their use is due to their narrow bandwidth and to their inherent nature as resonant devices. Many efforts have been made to overcome such deficiencies, including the use of thick substrates, cutting slots in the metallic patch and introducing parasitic patches either on the same layer or on top of the main patch. Aperture coupled stacked patch antennas have also been investigated, however, such devices also have certain drawbacks.
In many applications, such as phased array radars, low profile antennas are required and bandwidths less than a few percent are acceptable. Therefore, microstrip antennas have many desirable features. The microstrip antenna is constructed on a thin dielectric sheet using printed circuit board and etching techniques. The most common board is a dual copper coated polytetrafluoroethylene (Teflon) fiberglass as it allows the microstrip antenna to be curved to conform to the shape of the mounting surface. The patch antenna itself may be square, rectangular, round, elliptical and the like. The two most common geometries, rectangular and round, are widely employed. Circular polarized radiation can be obtained by exciting the square or round element at two feed points 90° (degrees) apart and in quadrature phase. A direct probe connected patch antenna element which is suitable for application at low UHF frequencies is required for a phased array application. The impedance matching of such an antenna should be compact, mechanically simple, and take advantage of the volume occupied by the patch antenna element. In the prior art a broad band antenna element requires the use of thick substrates with low relative dielectric constants approaching that of air. The direct probe connection fixed substrate geometry has a long probe length constituting a series inductance which must be compensated to allow wideband impedance matching. The prior art has employed a series compensation technique where a series capacitor at the end of the probe is formed instead of a direct connection to the patch. A plate is connected to the probe and the surface forms one plate of a capacitor with the patch being the other plate. The proximity of the plate to the patch sets the capacitance to the desired value to create a series resonant circuit at the frequency of operation. Thus, the input impedance is conjugate impedance matched to a real value. This prior art which utilizes a series resonant circuit for probe compensation has no direct DC connection to the patch. The open circuited probe combined with a small plate forms the required capacitor for series resonance. Multiple substrate layers are used with the plate embedded between the layers. The plate is mechanically inserted between the substrate layers and DC connected to the probe. This is difficult to provide. Furthermore, the prior art devices and methods encounter difficulty in meeting required frequency response for many applications. Still further, such prior art antennas are susceptible to breakdown at high transmission powers.